I-Hong Li

Ren-Kae Shiue

Shyi-Kaan Wu

Gold Bulletin

Springer - Publisher Connector

ORIGINAL PAPER
Infrared brazing of Fe3Al intermetallic compound
using gold-based braze alloy
Ren-Kae Shiue & Shyi-Kaan Wu & I-Hong Li
Published online: 22 February 2011
# The Author(s) 2011. This article is published with open access at Springerlink.com
Abstract Infrared-brazing Fe3Al with Au–44Cu as filler
metal has been investigated. The brazed joint consists
mainly of a β-phase, Au8−xCu4+xAl4, caused by the
dissolution of Al from Fe3Al substrate into the braze alloy.
The depletion of Al from Fe3Al substrate results in the
formation of a layer of β-phase particles dispersed in the
Fe-rich phase. The highest shear strength for AuCu filler is
327 MPa for specimens infrared brazed at 880°C for 180 s.
The brazed joint is mainly fractured along the central β-
phase in which the fractograph exhibits quasi-cleavage with
dimples. Increasing the brazing time or temperature will
deteriorate the bonding strength of the joint, and the
fracture mode is prone to cleavage of brittle fracture. Au–
44Cu filler demonstrates a great potential for bonding
Fe3Al intermetallic compound.
Keywords Au-based filler . Brazing .Wetting . Fe3Al
Introduction
Iron aluminides, including FeAl and Fe3Al, are featured
with low density, low cost, and good oxidation resistance
[1, 2]. The inherent brittleness of iron aluminides can be
improved by adding 2–5 wt.% Cr [1]. In recent years, iron
aluminides have been considered for structural applications.
The importance of joining iron aluminides have been
increasing in application of such alloys in industry.
Compared with welding, vacuum brazing has many
advantages, such as relatively lower joining temperature,
less oxidation, and lower distortion [3]. Fe3Al can be joined
by brazing without melting the base metal in order to retain
its mechanical properties [4, 5].
The selection of filler metal plays an important role in
brazing intermetallics, since the presence of brittle interme-
tallic phase in the joint may significantly impair their
bonding strength [6]. Both the wettability of the molten
braze on Fe3Al substrate, and the formation of intermetallic
phases in the joint should be taken into consideration.
Brochu et al. found that Fe3Al alloyed with Cr can be
wetted by Cu at 1,070°C, which is lower than the melting
point of pure Cu [7]. The brazing temperature of Fe3Al with
Cu filler metal is above 1,050°C, which can be decreased
by alloying Cu with Au. According to the Au–Cu binary
phase diagram, Cu is completely soluble in Au, and Au–
44Cu in atomic percent has the lowest melting temperature
of 910°C [8]. To the best of our knowledge, the brazing
behavior of Au–44Cu braze alloy on the Fe3Al substrate
has not been evaluated in detail. Therefore, it is chosen as
the braze alloy in this study.
Compared with traditional furnace brazing, infrared
brazing is better suited for studying the microstructural
evolution of the joint with the advantage of its rapid heating
rate being as high as 50°C/s [9, 10]. A short brazing cycle
does less damage to the base metal during infrared brazing.
This study focuses on the microstructural evolution and
bonding strength of the infrared brazed Fe3Al joints using
Au–44Cu as filler metal. The feasibility of infrared brazing
Fe3Al with Au-based braze alloy is also evaluated.
R.-K. Shiue : S.-K. Wu
Department of Materials Science and Engineering,
National Taiwan University,
Taipei 106, Taiwan
S.-K. Wu (*) : I.-H. Li
Department of Mechanical Engineering,
National Taiwan University,
Taipei 106, Taiwan
e-mail: skw@ntu.edu.tw
Gold Bull (2011) 44:49–56
DOI 10.1007/s13404-011-0007-8
Experimental procedure
The base metal used in the experiment was Fe3Al plate
with the composition of Fe–28Al–2Cr (atomic percent).
All joined surfaces were ground by SiC papers up to grit
1,200 and then ultrasonically cleaned by acetone prior to
infrared brazing. Au–44Cu (atomic percent) foil of 50 μm
thick was used as the brazing filler metal. The heating rate
of infrared furnace was kept at 10°C/s under the vacuum
of 5×10−5 mbar. The brazing temperature was 870°C,
880°C, and 900°C, and the brazing time was 180 s and
300 s.
Dynamic wetting angle on the Fe3Al substrate was
measured by the sessile drop test using a filler ball,
approximately 0.15 g, prepared by vacuum arc remelting
0
20
40
60
80
100
120
140
160
180
0 50 100 150 200 250 300
Time (s)
W
et
tin
g 
an
gl
e 
(dr
gre
e)
 860 C
 880 C
 910 C
 920 C
 930 C
Fig. 1 Dynamic wetting angle measurements of Au–44Cu filler on
Fe3Al substrate under various brazing conditions
at% A B C D E 
Au 43.1 44.1 0.8 5.9 1.1 
Cu 32.0 32.1 1.6 4.2 1.4 
Fe 2.7 2.7 85.1 76.0 85.2 
Al 20.4 20.5 10.2 10.7 10.1 
Cr 1.8 0.6 2.4 3.1 2.2 
phase β-phase β-phase Fe-rich Fe-rich Fe-rich 
A
D
C
B
E
Fig. 2 EPMA BEIs and WDS
chemical analysis results of the
Au–44Cu/Fe3Al wetting speci-
men at 880°C for 300 s
50 Gold Bull (2011) 44:49–56
[6, 10]. The ULVAC Sinko-Riko RHL-P816C infrared
furnace was used as the main body of the vacuum furnace.
The dynamic wetting angle was performed in a vacuum of
5×10−5 mbar. Infrared rays were generated from infrared
lamps, transmitted through a transparent quartz tube and
focused on the specimen holder. One end of the quartz tube
was attached to the specimen holder, and the other end was
designed to transmit the signal into the CCD image
recording system. The filler metal was placed on the
substrate (Fe3Al), and a thermocouple was simultaneously
contacted with the substrate. A graphite holder was
positioned under the substrate to enhance the absorptivity
of infrared rays.
Shear tests were performed to evaluate the bonding
strength of the brazed joint [10]. The specimen was
compressed by a Shimadzu AG-10 universal testing
machine with a constant speed of 0.0167 mm/s. The
experimental data were averaged from at least two measure-
ments for each brazing condition. Cross sections of joints
and their fractographs after shear tests were examined by a
JEOL JXA 8600SX electron probe microanalyzer (EPMA)
equipped with the wavelength dispersive spectroscope
(WDS) using the operation voltage of 15 kV and the
minimum spot size of 1 μm.
Results and discussion
Figure 1 shows dynamic wetting angle measurements of
Au–44Cu filler on Fe3Al substrate under various temper-
atures between 0 and 300 s. As can be seen, the wetting
curves of Au–44Cu filler on Fe3Al substrate at 920 and
930°C are almost identical, and a similar wetting behavior
is observed for the specimen tested at 910°C. Since the
melting point of Au–44Cu braze alloy is 910°C, the
molten braze alloy wets readily the Fe3Al substrate in
50 s, and the wetting angle is stabilized below 5°. The
wetting behavior of Au–44Cu braze at 880°C is different
from that above 910°C. For the specimen tested at 880°C,
the initial wetting angle is kept at 155° until 20 s, dropped
into 12° in 80 s, and finally stabilized at 6° between 100
and 300 s. The wetting curve tested at 860°C shows poor
wetting angle of 33° even if the time period is extended to
300 s.
As seen in Fig. 1, the braze alloy still wets effectively
the Fe3Al substrate at 880°C, below the melting point of
Au–44Cu. It is deduced that the dissolution of Al from the
Fe3Al substrate into the braze alloy causes decrease in
melting point of braze alloy. According to the Al–Au
binary alloy phase diagram, the solidus temperature of Au
is significantly decreased from 1,064.4°C (pure Au) to
545°C (Au–16Al, atomic percent) [9]. Figure 2 shows
EPMA backscattered electron images (BEIs) and WDS
chemical analysis results of the Au–44Cu/Fe3Al wetting
specimen tested at 880°C for 300 s. There are at least two
phases identified from the WDS chemical analysis. The
braze alloy consists of 43.1Au–32.0Cu–2.7Fe–20.4Al–
1.8Cr in atomic percent as marked by A in Fig. 2. Because
the braze alloy consists primarily of Al, Au, and Cu, the
Al–Au–Cu ternary alloy phase diagram is cited here in
order to explain the phase evolution of the brazed
specimen.
Figure 3 displays the isothermal section of the Al–Au–
Cu ternary alloy phase diagram in atomic percent [11, 12].
The chemical composition of area A in Fig. 2 is close to the
β-phase [11]. The β-phase has a nominal stoichiometry of
Au8-xCu4+xAl4, where for example in Spangold X≒1. The
β-phase stiochiometries for 0<×<1 in Au8−xCu4+xAl4 will
be completely molten on the 750°C isothermal section. The
β-phase (Au7Cu5Al4) has a solidus temperature of 726°C
and liquidus temperature of 767°C, which are much lower
than the testing ones. It is obvious that the dissolution of Al
from the Fe3Al substrate into the Au–44Cu braze results in
formation of the β-phase melt at 880°C. Accordingly, the
wetting of Au–44Cu braze alloyed with Al on the substrate
is greatly enhanced even below the melting point of Au–
44Cu. The depletion of Al from the Fe3Al substrate also
results in formation of the Fe-rich phase as marked by C in
Fig. 2. Additionally, a blocky β-phase in the Fe-rich phase
as marked by B- and Fe-rich precipitates in the β-phase as
marked by D, as seen in Fig. 2, are also formed due to the
limited solubility between the β-phase and Fe-rich phase at
room temperature.
Figure 4 illustrates EPMA BEIs and WDS chemical
analysis results of the Au–44Cu/Fe3Al wetting specimen at
Fig. 3 Isothermal section of the Al–Au–Cu ternary alloy phase
diagram in atomic percent [11, 12]
Gold Bull (2011) 44:49–56 51
860°C for 300 s. The Fe-rich phase as marked by C is still
seen in the figure. However, the chemical composition of
the braze alloy is different from that of the β-phase. In
Fig. 4, the Al content in area A is 12.6 at.%, so it is
identified as the α-phase according to Fig. 3 [11, 12]. It is
also noted that a blocky β-phase is found in the interfacial
Fe-rich phase as marked by B in the figure. Dissolution of
Al from the substrate into the braze alloy is greatly reduced
with decreasing temperature, while the melting point of the
braze alloy cannot be significantly decreased by alloying
sufficient Al in the Au–440Cu melt. For the specimen
tested at 860°C for 300 s, the wetting angle is above 30o,
which is insufficient to wet Fe3Al during brazing.
Figure 5 displays cross sections of EPMA BEIs and
WDS chemical analysis results of Au–44Cu/Fe3Al speci-
mens infrared brazed under various conditions. Micro-
structures of infrared brazed joints are quite similar. The
brazed joints consist primarily of the β-phase, and β-phase
particles are widely dispersed in the interfacial Fe-rich
phase.
The corrosion resistance of the brazed joint should be
considered for industrial application. For the multi-phase
at% A B C 
Au 48.2 48.2 1.1 
Cu 34.3 28.7 2.1 
Fe 4.3 3.5 85.7 
Al 12.6 18.7 8.5 
Cr 0.7 0.9 2.4 
phase α-phase β-phase Fe-rich 
A
CB
Fig. 4 EPMA BEIs and WDS
chemical analysis results of the
Au–44Cu/Fe3Al wetting speci-
men at 860°C for 300 s
52 Gold Bull (2011) 44:49–56
structure consisting of highly noble and non-noble phases
may be highly susceptible to corrosion. Based on the
experimental observation, the brazed joint is mainly
composed of β-phase matrix, and β-phase particles are
widely dispersed in the interfacial Fe-rich phase. The β-
phase is alloyed with high content of Au, so it is
considered as a noble phase. In contrast, the interfacial
Fe-rich phase primarily alloyed with 8–16 at.% Al is a less
noble phase. Accordingly, the brazed joint is susceptible
interfacial corrosion in application, and needs further
study.
Table 1 shows shear test results of different brazing
conditions. Shear strength of the joint infrared brazed at
870°C for 180 s is 320 MPa. The specimen infrared brazed
at 880°C for 180 s has the maximum shear strength of
327 MPa. Maximum shear strengths using other alternative
at% A B C D E F G H 
Au 40.8 0.8 39.9 37.6 40.3 0.9 40.3 0.9 
Cu 28.3 1.8 28.2 29.3 30.0 0.8 30.0 0.8 
Fe 2.3 79.8 7.5 3.7 4.7 79.4 4.7 79.4 
Al 26.8 15.2 23.1 27.3 23.4 16.3 23.4 16.3 
Cr 1.9 2.3 1.3 2.1 1.7 2.6 1.7 2.6 
phase β-phase Fe-rich β-phase β-phase β-phase Fe-rich β-phase Fe-rich 
(c)
E
F
(d)
G
H
(a)
A
B C
D
(b)
Fig. 5 Cross sections of EPMA BEIs and WDS chemical analysis results of the Au–44Cu/Fe3Al specimens infrared brazed at a 870°C for 180 s,
b 880°C for 180 s, c 880°C for 300 s, and d 900°C for 180 s
Gold Bull (2011) 44:49–56 53
brazes are lower than that using Au–44Cu braze. For
example, maximum shear strengths of the brazed joints are
127 MPa using pure Ag braze, 181 MPa using 72Ag–28Cu
(weight percent) braze and 291 MPa using pure Cu braze,
respectively [4, 13]. The application of Au–44Cu braze in
brazing Fe3Al demonstrates the highest shear strength
among alternative brazes. Besides, the brazing temperature
of Au–44Cu braze is much lower than those of pure Ag and
Cu brazes.
Figure 6 illustrates EPMA cross sections and fracto-
graphs of Fe3Al specimens after shear test of the
joint infrared brazed at 870°C and 880°C for 180 s,
respectively. Both cracks are initiated and propagated
along the β-phase as displayed in Figs. 6a, c. As seen in
Figs. 6b, d, fractured surfaces are featured with quasi-
cleavage with dimples.
Further increasing the infrared brazing temperature or
time results in deterioration of shear strength of the joint
below 240 MPa. Because the chemical composition of β-
phases as marked by A, D, E, and G in Fig. 5 are
approximately the same, the fracture mechanism of higher
brazing temperature/longer brazing time is different from
that of lower temperature/shorter time. Figure 7 illustrates
(a) (b)
(c) (d)
Fe-rich Fe-richCrack
Fe-rich Fe-richCrack
Fig. 6 EPMA cross sections and fractographs of specimens after shear test of the joint infrared brazed at a, b 870°C for 180 s and c, d 880°C for
180 s
Table 1 Summary of shear test results for different brazing conditions
Brazing temperature (°C) Brazing time (s) Shear strength (MPa)
870 180 320
880 180 327
880 300 210
900 180 233
54 Gold Bull (2011) 44:49–56
the grain-boundary penetration of the braze alloy after
infrared brazing under various brazing conditions. It is
obvious that the Au-based melt penetrates along the grain
boundaries of Fe3Al substrate. It is consistent with previous
reports of wetting the Fe3Al substrate by pure Cu [7, 14].
The dissolution of Al from the Fe3Al substrate into the Cu
melt results in formation of the interfacial Fe-rich layer.
Lattice constants of Fe3Al with DO3 structure and α-Fe are
0.578 and 0.298 nm, respectively. The formation of the Fe-
rich phase next to the Fe3Al substrate leads to decrease in
substrate volume during brazing, and the molten braze
penetrates the substrate along its grain boundary. The effect
of grain-boundary penetration is enhanced by increasing the
brazing temperature and/or time as displayed in Fig. 7.
Figure 8 shows the EPMA cross section and fractographs
of Fe3Al after shear test of the joint infrared brazed at 880°C
for 300 s. As can be seen, the presence of β-phase at the
grain boundary weakens greatly the strength of the Fe3Al
substrate. Cracks propagate along the grain-boundary β-
phase as indicated by an arrow in Fig. 8a. Fractograph of the
fractured surface shows the concave Fe3Al substrate
(Fig. 8b), and cleavage-dominated fracture is widely observed
from the fractured β-phase in the brazed zone (Fig. 8c).
Conclusions
Wetting, microstructural evolution and shear strength of
infrared brazed Fe3Al intermetallic compound using Au–
44Cu (atomic percent) braze alloy have been investigated.
The braze alloy demonstrates excellent wettability of the
Fe3Al substrate above 910°C. Wettability of the Au–44Cu
(a)
braze
Fe3AI
(c)
brazeFe3AI
Fe3AI
(d)
braze
Fe3AI
Fe3AI
(b)
braze
Fe3AI Fe3AI
Fig. 7 Grain-boundary penetration of the braze alloy after infrared brazing at a 870°C for 180 s, b 880°C for 180 s, c 880°C for 300 s, and
d 900°C for 300 s
Gold Bull (2011) 44:49–56 55
braze is impaired with decreasing temperature due to
insufficient dissolution of Al from the Fe3Al substrate into
the braze. The brazed joint consists mainly of β-phase,
Au8-xCu4+xAl4, caused by the dissolution of Al from the
Fe3Al substrate into the braze alloy. Meanwhile, depletion
of Al from the Fe3Al substrate results in the formation of a
layer of β-phase particles dispersed in the Fe-rich matrix.
The specimen infrared brazed at 880°C for 180 s has the
maximum shear strength of 327 MPa. Further increasing
the infrared brazing temperature or time leads to deteriorate
the shear strength of the joint below 240 MPa. The
presence of grain-boundary β-phase along the Fe3Al
substrate is attributed to decreasing strength of the Fe3Al
substrate. Cracks propagate along the grain-boundary β-
phase, and cleavage-dominated fracture is widely observed
from fractographs of the brazed zone. Infrared brazing
using Au–44Cu filler metal shows potential application in
joining Fe3Al intermetallic compound.
Acknowledgments The authors gratefully acknowledge the finan-
cial support of this study from the National Science Council (NSC),
Republic of China, under grant no. 97-2221-E002-037-MY3.
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use,
distribution and reproduction in any medium, provided the original
author(s) and source are credited.
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(a)
(b)
(c)
Fe3AI
Concave Fe3AI
Fe3AICrack
braze braze
Crack
Fig. 8 a Cross section and b, c EPMA fractographs of the specimen
after shear test of the joint infrared brazed at 880°C for 300 s
56 Gold Bull (2011) 44:49–56


Infrared brazing of Fe3Al intermetallic compound using gold-based braze alloy

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Infrared brazing of Fe3Al intermetallic compound using gold-based braze alloy

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